1. Field of the Invention
Some preferred embodiments of the present invention relate, inter alia, to a differential comparator configured to suppress stability deterioration, and an image A/D converter, especially a pipeline type A/D converter equipped with the differential comparator.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
In a conventional semiconductor integrated circuit, in cases where a DC input is continuously inputted into an A/D converter including a differential comparator for a long time, a phenomenon called “stability” occurs. In general, the stability is one of deterioration phenomena of MOS transistor characteristics. When a strong gate electric field generated at the gate oxide film of a MOS transistor is maintained for a long time or when the strong gate electric field is maintained in a dynamical manner, electric charges will be trapped by defects of the gate oxide film, resulting in temporal changes of the transistor characteristics, which in turn mainly causes changes of the threshold voltage Vt. This phenomenon is called “stability.” Especially, the stability likely causes an offset in a differential circuit. For example, in a pipeline type A/D converter for performing A/D conversions using a differential circuit, a long time impression of a certain level of a DC voltage causes miscoding.
In a differential amplifier mounted in a pipeline A/D converter, the latch circuit for latching an output from the differential amplifier will hold the output of the same value for a long time when a DC voltage of a certain level is impressed from the analog input for a long time. This may cause stability deterioration of the MOS transistor forming the latch circuit.
As will be understood from the above, if a DC input is inputted into an A/D converter including a differential comparator for a long time, the DC stress of the inner latch circuit is fixed in one direction, which tends to cause stability deterioration. As a result, an offset occurs. When such offset occurs, accurate A/D conversions cannot be expected, resulting in malfunctions or causing various problems.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.